Gas diffusion electrode, solid polymer electrolyte membrane, method of producing them, and solid polymer electrolyte type fuel cell using them

ABSTRACT

A gas diffusion electrode for solid polymer electrolyte fuel cell comprising a gas diffusion electrode and a catalyst layer, characterized in that said catalyst layer is provided with a catalytic substance and an ion exchange resin having pores. In this arrangement, a three-phase boundary can be provided also inside the catalyst layer while maintaining sufficient electron conductivity, capability of supplying reaction gas and protonic conductance, making it possible to enhance the polarization characteristics and power density thereof.

TECHNICAL FIELD

[0001] The present invention relates to a solid polymer electrolyte fuelcell.

BACKGROUND ART

[0002] A solid polymer electrolyte fuel cell has a structure comprisinga gas diffusion electrode provided on both surfaces of an ion exchangemembrane (solid polymer electrolyte). This is an apparatus which allowsthe electrochemical reaction of an oxidizing agent such as oxygen with afuel such as hydrogen to give electric power.

[0003] A gas diffusion electrode comprises a catalyst layer and a gasdiffusion layer. The catalyst layer is formed by binding a catalyst suchas particulate noble metal and/or carbon powder having a particulatenoble metal supported thereon with a binder or the like. As such abinder there is normally used a fluorinic resin such aspolytetrafluoroethylene (PTFE). This fluorinic resin is also a waterrepellent which renders a catalyst layer properly water-repellent. As agas diffusion layer there is used a water repellent carbon paper or thelike.

[0004] The characteristics of such a solid polymer electrolyte fuel celldrastically depend on the structure of the gas diffusion electrode,particularly catalyst layer. In other words, the electrode reactionproceeds on a three-phase boundary where the catalyst in the catalystlayer, the electrolyte and oxygen or hydrogen are present. However,since this type of a fuel cell comprises a solid material aselectrolyte, this three-phase boundary is limited to a two-dimensionalboundary of electrolyte with catalyst layer, lowering the activity ofthe gas diffusion electrode. Attempts have been made so far to enhancethe activity of gas diffusion electrode by various methods forincreasing three-phase boundary.

[0005] One of these methods is to increase the surface of solid polymerelectrolyte membrane and hence its area of contact with a catalyst. Forexample, JP-A-58-7423 (The term “JP-A” as used herein means an“unexamined published Japanese patent application”) proposes a processfor the production of a porous polymer electrolyte membrane. However,the above cited patent application has no reference to effects andcharacteristics developed when this production method is applied to fuelcell. Further, JP-A-4-169069 proposes a method involving the rougheningof the surface of a solid polymer electrolyte membrane by sputtering orlike means.

[0006] Another method is to incorporate an ion exchange resin in acatalyst layer and hence increase its area of contact with a catalyst.For example, JP-B-62-61118 (The term “JP-A” as used herein means an“unexamined published Japanese patent application”) and JP-B-62-61119propose a method which comprises the preparation of a catalyst layerfrom a mixture obtained by adding an ion exchange resin solution to acatalytic substance. JP-A-4-162365 employs a method involving thecoating of the surface of a catalytic substance by an ion exchange resinsolution. JP-B-2-48632 and JP-A-6-333574 propose a method whichcomprises spraying or applying an ion exchange resin solution to acatalyst layer, and then drying the catalyst layer to provide thecatalyst layer with an ion exchange resin. Further, JP-A-7-183035proposes a method which allows a catalytic substance to adsorb an ionexchange resin colloid.

[0007] On the other hand, another factor affecting the characteristicsof a solid polymer electrolyte fuel cell is the electrical conductanceof solid polymer electrolyte membrane. In other words, in order toenhance the power of a solid polymer electrolyte fuel cell, it isimportant to lower the resistance of solid polymer electrolyte membrane.To this end, a method involving the provision of a solid polymerelectrolyte membrane having a reduced thickness or a method involvingthe increase of the amount of sulfonic group incorporated in ionexchange resin has been proposed.

[0008] Although the foregoing conventional methods can increase thesurface area of the ion exchange resin membrane in the catalyst layeritself, it can hardly fill the pores in the catalyst layer or thevalleys of unevenness on the catalyst layer with a catalytic substancesuch as particulate carbon having a particulate noble metal catalystsupported thereon. Thus, it is extremely difficult to increasethree-phase boundary by these methods.

[0009] Further, a method is disclosed which comprises covering acatalytic substance such as particulate carbon having a particulatenoble metal catalyst supported thereon to form a catalyst layer havingan increased contact area and hence an increased three-phase boundary asmentioned above. In this case, it is indispensable to leave thecatalytic substance partly uncovered by using a water repellent such asPTFE to improve the properties of fuel cell or form a uniform thincoating film to enhance the gas permeability thereof. If a portion leftuncovered is formed, a portion which is excessively covered or entirelyuncovered due to the positional relationship between catalyticsubstances is formed, lowering the gas permeability or making itimpossible to obtain the desired activity. Accordingly, the resultingfuel cell exhibits deteriorated properties. Further, if the methodinvolving the formation of a uniform thin coating film is employed, itis extremely difficult to form such a uniform thin coating film,deteriorating the productivity. In addition, if the thickness of thecoating film is reduced, the path of migration of proton is remarkablyreduced, deteriorating the properties of the fuel cell.

[0010] In addition, if the thickness of the coating film is reduced, thepath of migration of proton is remarkably reduced, deteriorating theproperties of the fuel cell.

[0011] Therefore, the present invention has been worked out to givesolution to the foregoing prior art problems. An object of the presentinvention is to provide a gas diffusion electrode and a solid polymerelectrolyte which increase three-phase boundary in the catalyst layerwhile sufficiently securing the path of migration of substances such asoxygen, hydrogen and produced water all over the catalyst layer and thecatalytic substance without deteriorating the ionic conductivity thereofin a solid polymer electrolyte fuel cell and a high power solid polymerelectrolyte fuel cell comprising the gas diffusion electrode and solidpolymer electrolyte. Another object of the present invention is toprovide a process for the production of a gas diffusion electrode whichcan also secure electrical contact with a catalytic substance.

DISCLOSURE OF THE INVENTION

[0012] The foregoing objects of the present invention are accomplishedby the following inventions.

[0013] The first invention concerns a gas diffusion electrode for solidpolymer electrolyte fuel cell comprising a gas diffusion layer and acatalyst layer, characterized in that said catalyst layer is providedwith a catalytic substance and an ion exchange resin having pores.

[0014] The second invention concerns a gas diffusion electrode for solidpolymer electrolyte fuel cell comprising a gas diffusion layer and acatalyst layer, characterized in that said catalyst layer is providedwith an ion exchange resin having pores having a diameter of from 0.05to 5.0 μm and porosity of not less than 40%.

[0015] The third invention concerns a gas diffusion electrode for solidpolymer electrolyte fuel cell comprising a gas diffusion layer and acatalyst layer, characterized in that said ion exchange resin is aperfluorosulfonic acid resin and said catalytic substance is orcomprises a particulate noble metal or carbon having a particulate noblemetal supported thereon.

[0016] The fourth invention, which is according to the first, second orthird invention, concerns to a process which comprising forming an ionexchange resin coating film, made of a solution obtained by dissolvingan ion exchange resin in a solvent containing an alcohol, on a catalystlayer precursor with the ion exchange resin coating film on it in anorganic solvent having a polar group other than alcoholic hydroxyl groupso that the ion exchange resin in solidified and rendered porous. In thepresent invention, the coating film may be in the form of membrane ormay comprise a catalytic substance incorporated in an ion exchangeresin. To be short, it suffices if the ion exchange resin is presentaround the catalytic substance.

[0017] The fifth invention concerns a solid polymer electrolytemembrane-gas diffusion electrode assembly comprising a gas diffusionelectrode for solid polymer electrolyte fuel cell according to any oneof the first to third inventions provided on at least one side of asolid polymer electrolyte membrane.

[0018] The sixth invention concerns a solid polymer electrolyte fuelcell comprising a solid polymer electrolyte membrane-gas diffusionelectrode assembly according to the fifth invention.

[0019] The seventh invention concerns a solid polymer electrolytemembrane comprising an ion exchange resin as a constituent and havingpores.

[0020] The eighth invention concerns a solid polymer electrolytemembrane according to the seventh invention, wherein said solid polymerelectrolyte membrane has a pore diameter of from 0.02 to 1.0 μm and aporosity of not less than 10%.

[0021] The ninth invention concerns a solid polymer electrolyte membraneaccording to the seventh or eighth invention, wherein said ion exchangeresin is a perfluorosulfonic acid resin.

[0022] The tenth invention, which is according to the seventh, eighth orninth invention, concerns a process for the production of a solidpolymer electrolyte membrane which comprises soaking a solutioncomprising an ion exchange resin dissolved in a solvent containing analcohol in an organic solvent having a polar group other than alcoholichydroxyl group so that said ion exchange resin is solidified andrendered porous to form an ion exchange resin membrane having pores.

[0023] The eleventh invention concerns a solid polymer electrolyte fuelcell comprising a solid polymer electrolyte membrane according to theseventh, eighth or ninth invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is an electron microphotograph illustrating the surfaceconditions of an ion exchange resin having pores according to thepresent invention.

[0025]FIG. 2 is an electron microphotograph illustrating the surfaceconditions of another ion exchange resin having pores according to thepresent invention.

[0026]FIG. 3 is an electron microphotograph illustrating the surfaceconditions of a further ion exchange resin having pores according to thepresent invention.

[0027]FIG. 4 is a specific sectional view of a solid polymerelectrolyte-gas diffusion electrode assembly according to the presentinvention.

[0028]FIG. 5 is a specific sectional view of an ion exchange resin 1having pores according to the present invention.

[0029]FIG. 6 is a chart illustrating the process for the preparation ofa gas diffusion electrode comprising a catalyst layer provided with anion exchange resin having pores according to the present invention.

[0030]FIG. 7 is an electron microphotograph illustrating the surfaceconditions of a catalyst layer in a gas diffusion electrode providedwith an ion exchange resin having pores according to the presentinvention.

[0031]FIG. 8 is a graph illustrating the current density-cell voltagecharacteristic curve of cells A, B, C and D according to the presentinvention and a prior art known cell E.

[0032] In FIGS. 4 and 5, the reference numeral 1 indicates an ionexchange resin having pores, the reference numeral 2 indicates acarbon-supported platinum catalyst, the reference numeral 3 indicates apolytetrafluoroethylene (binder), the reference numeral 4 indicates agas diffusion layer, the reference numeral 5 indicates a catalyst layer,the reference numeral 7 indicates a solid polymer electrolyte membrane,the reference numeral 8 indicates a void, the reference numeral 9indicates a solid polymer electrolyte membrane-gas diffusion electrodeassembly, and the reference numeral 11 indicates a pore.

[0033] Best Embodiments of Implication of the Invention

[0034] A process for the production of this electrode will be describedhereinafter. In other words, a solution comprising an ion exchange resindissolved in a solvent containing an alcohol is soaked in an organicsolvent having a polar group other than alcoholic hydroxyl group such asbutyl acetate so that the ion exchange resin dissolved therein issolidified and rendered porous.

[0035] As the ion exchange resin solution there may be used a 5 wt-%Nafion solution (produced by Aldrich Inc. of USA), which is acommercially available perfluorosulfonic acid resin solution. ThisNafion solution may be partly concentrated to give variousconcentrations. The Nafion solution which has been thus adjusted inconcentration is applied to a glass plate which is then soaked in butylacetate and allowed to stand. The glass plate is then allowed to dry atroom temperature to form an ion exchange resin coating film having poresthereon. Nafion is a registered trademark of Du Pont Inc.

[0036]FIGS. 1 and 2 each are an example of diagram (electronmicrophotograph) illustrating the surface conditions of an ion exchangeresin having pores thus prepared from a 9 wt-% Nafion solution and a 13wt-% Nafion solution, respectively.

[0037] As can be seen in both the two drawings, this is a porous ionexchange resin having a three-dimensional network structure in whichcontinuous pores are formed. The diameter of the pores thus formed orthe porosity of the porous ion exchange resin varies with theconcentration of the ion exchange resin solution. In other words, thehigher the concentration of the ion exchange resin solution is, thesmaller are the diameter of the pores thus formed and the porosity ofthe porous ion exchange resin. On the contrary, the lower theconcentration of the ion exchange resin solution is, the greater are thediameter of the pores thus formed and the porosity of the porous ionexchange resin.

[0038] By using this process, the gas diffusion electrode according tothe present invention can be prepared. In other words, a powder layermade of a catalytic substance alone or a material obtained by binding aparticulate catalytic substance with a binder, i.e., catalyst layerprecursor is prepared. This catalyst layer precursor, e.g., materialobtained by binding a particulate catalytic substance with a binder isthen soaked in a solution obtained by dissolving an ion exchange resinin a solvent containing an alcohol. Alternatively, this solution isapplied to the surface of the catalyst layer precursor. Thus, a coatinglayer is formed on the catalyst layer precursor. The catalyst layerprecursor is then soaked in an organic solvent having a polar groupother than alcoholic hydroxyl group such as butyl acetate so that theion exchange resin thus coated is solidified and rendered porous to forman ion exchange resin having pores open to the catalytic substance inthe catalyst layer. If the catalyst layer precursor is a catalyst powderlayer, the foregoing solution is allowed to penetrate into the powderlayer to form a coating membrane. The coating film made of this ionexchange resin solution may be in the form of membrane or may comprise acatalytic substance incorporated in an ion exchange resin.

[0039] As the organic solvent having a polar group other than alcoholichydroxyl group there may be used an organic solvent having a C₁₋₇ carbonchain having an ester group in its molecule such as propyl formate,butyl formate, isobutyl formate, ethyl acetate, propyl acetate,isopropyl acetate, allyl acetate, butyl acetate, isobutyl acetate,pentyl acetate, isopentyl acetate, methyl propionate, ethyl propionate,propyl propionate, methyl acrylate, butyl acrylate, isobutyl acrylate,methyl butyrate, isopropyl isobutyrate, 2-ethoxyethyl acetate and2-(2-ethoxyethoxy) ethyl acetate, singly or in admixture, an organicsolvent having a C₃₋₅ carbon chain having an ether group in its moleculesuch as dipropyl ether, dibutyl ether, ethylene glycol dimethyl ether,ethylene glycol diethyl ether, tripropylene glycol monomethyl ether andtetrahydrofuran, singly or in admixture, an organic solvent having aC₄₋₈ carbon chain having a ketone group in its molecule such as methylbutyl ketone, methyl isobutyl ketone, methyl hexyl ketone and dipropylketone, singly or in admixture, an organic solvent having a C₁₋₅ carbonchain having an amine group in its molecule such as isopropylamine,isobutylamine, tertiary butylamine, isopentylamine and diethylamine,singly or in admixture or an organic solvent having a C₁₋₆ carbon chainhaving a carboxyl group in its molecule such as propionic acid, valericacid, caproic acid and heptanoic acid, singly or in admixture.

[0040] The perfluorosulfonic acid resin solution has been described withreference to commercially available Nafion solution. However, thepresent invention is not limited to this Nafion solution. Anyperfluorosulfonic acid solution may be used.

[0041] A second process for the production of the gas diffusionelectrode will be described hereinafter.

[0042] A mixed dispersion of an ion exchange resin solution with asolution of a second polymer compound incompatible therewith isprepared. The mixed dispersion is then applied to a catalyst layerprecursor formed by constituents such as catalytic substance and binder,e.g., by coating the catalyst layer precursor with the mixed dispersion.The catalyst layer precursor is then dried so that the solvent isremoved from the mixed dispersion to form a coating film having the ionexchange resin and the second polymer compound in phase-separated state.The coating film thus phase-separated is then dipped in a solvent whichcannot dissolve the ion exchange resin but the second polymer compoundtherein so that the second polymer compound is eluted. Thus, pores areformed where the second polymer compound is eluted. In this manner, anion exchange resin having pores in a catalyst layer is formed.

[0043] A schematic sectional view of a solid polymer electrolytemembrane-gas diffusion electrode assembly of the present inventionprovided with an ion exchange resin having pores in a catalyst layer isshown in FIG. 4. In this diagram, the reference numeral 1 indicates anion exchange resin having pores. The reference numeral 2 indicates acatalytic substance which is a carbon-supported platinum catalyst havinga particulate noble metal such as particulate platinum supported as acatalyst on a particulate carbon. The reference numeral 3 indicates apolytetrafluoroethylene as a binder. The reference numeral 5 indicates acatalyst layer formed by a carbon-supported platinum catalyst 2, abinder 3 and an ion exchange resin 1 having pores.

[0044] Formed in the structure formed by the carbon-supported platinumcatalyst 2 and the binder 3 in the catalyst layer, 5 are voids 8. Thecarbon-supported platinum catalyst 2 is provided with an ion exchangeresin 1 having pores on the surface thereof. The reference numeral 4indicates a gas diffusion layer which is a carbon paper provided withwater repellency. The gas diffusion electrode 6 is formed by the gasdiffusion layer 4 and the catalyst layer 5. The reference numeral 7indicates an ion exchange membrane which is a solid polymer electrolytemembrane. The gas diffusion electrode 6 is bonded to the ion exchangemembrane 7 to form a solid polymer electrolyte membrane-gas diffusionelectrode assembly 9.

[0045]FIG. 5 is an enlarged view illustrating a schematic section of theion exchange resin 1 having pores of FIG. 4. Formed on thecarbon-supported platinum catalyst 2 is the ion exchange resin 1 havingthree-dimensional network pores shown by the electron microphotographillustrating surface conditions in FIG. 1.

[0046] Referring to the process for the production of a gas diffusionelectrode according to the present invention, a process is preferablyused which comprises forming a coating film of an ion exchange resin 1on a catalyst layer precursor previously formed by a carbon-supportedplatinum catalyst 2 and a binder 3, and then forming three-dimensionalpores therein. In this manner, the carbon-supported platinum catalystparticles come in contact with each other with no ion exchange resinpresent interposed therebetween, making it possible to keep sufficientelectrical contact between the carbon-supported platinum catalystparticles while forming a sufficient path of migration of electron inthe catalyst layer. Further, since the ion exchange resin has pores, thecarbon-supported platinum catalyst is not excessively covered. Moreover,since these pores are continuous, the resulting catalyst layer exhibitsa high gas permeability that facilitates the supply of a gas such asoxygen or hydrogen to the catalyst portion. Further, since the ionexchange resin has a continuous three-dimensional network structure, asufficient path of migration of proton is formed.

[0047] Thus, sufficient electronic conduction, supply of reaction gasand protonic conduction can be provided, and a three-phase boundary canbe formed deep into the catalyst layer.

[0048] Accordingly, reactions represented by the following formulae canproceed also inside the catalyst layer. Thus, the substantial reactionarea can be increased, making it possible to obtain a gas diffusionelectrode having a high activity.

[0049] On the anode:

2H₂→4H⁺+4e ⁻

[0050] On the cathode:

O₂→4H⁺+4e ⁻2H₂O

[0051] In addition, as can be seen in the following examples, the effectof the present invention can be exerted remarkably because of the highconductance of the ion exchange resin having pores.

[0052] (Experiment 1)

[0053] An ion exchange resin having pores according to the presentinvention was prepared. The ion exchange resin thus prepared was thenmeasured for conductance.

[0054] A 5 wt-% Nafion solution, which is a commercially availableperfluorosulfonic acid resin solution, was heated to a temperature of60° C. with stirring so that the solvent was removed to prepare 9 wt-%,13 wt-% and 21 wt-% Nafion solutions.

[0055] The 9 wt-% Nafion solution was then applied to a glass platethrough a 300# (mesh) screen. The 13 wt-% and 21 wt-% Nafion solutionswere then each applied to a glass plate using a doctor blade with a gapadjusted to 0.1 mm.

[0056] These coated materials were then immediately soaked in n-butylacetate for 15 minutes. Subsequently, these coated materials werewithdrawn to the atmosphere where they were then allowed to dry at roomtemperature. As a result, a Nafion resin (ion exchange resin) membranehaving pores was formed on the glass plate. The ion exchange resinmembranes having pores prepared from the 9 wt-%, 13 wt-% and 21 wt-%Nafion solutions will be hereinafter referred to as “Porous membrane A”,“Porous membrane B”, and “Porous membrane C”, respectively. The diagram(electron microphotograph) illustrating the surface conditions of Porousmembranes A, B and C are shown in FIGS. 1, 2 and 3, respectively. Porousmembranes A, B and C exhibited a porosity of 90%, 70% and 40%,respectively.

[0057] For comparison, the foregoing 9 wt-%, 13 wt-% and 21 wt-% Nafionsolutions were applied to a glass plate through a screen or by using adoctor blade in the same manner as mentioned above, and then directlyallowed to dry at room temperature to form a membrane on the glassplate. The ion exchange resin membranes having pores prepared from the 9wt-%, 13 wt-% and 21 wt-% Nafion solutions will be hereinafter referredto as “Comparative membrane A”, “Comparative membrane B”, and“Comparative membrane C”, respectively.

[0058] Porous membranes A, B and C and Comparative membranes A, B and Cwere then measured for conductance in the following manner.

[0059] The six samples, i.e., Porous membranes A, B and C andComparative membranes A, B and C were each cut into a size of 1.5 cmwide×3.5 cm long. These samples were each soaked in a 0.5 mol/l dilutedsulfuric acid overnight, and then thoroughly washed with purified waterto give protonic type samples. The protonation treatment and themeasurement of conductance were conducted with these samples integratedwith the glass plate. During the measurement of conductance, thesesamples were soaked in purified water having a conductance of not morethan 0.2 μS/cm at a temperature of 25° C. In this arrangement, theseporous membranes and comparative membranes were each measured forconductance along the surface thereof.

[0060] For the measurement of conductance, a dc four-terminal currentinterrupter method was employed. In some detail, as the voltagemeasurement terminal and the current introduction terminal there wereeach used a platinum wire having a diameter of 1 mm. The gap between thevoltage measurement terminals was 5 mm. The gap between the currentintroduction terminals was 15 mm. A DC pulse current was applied to thecurrent introduction terminal. The change of voltage developed betweenthe voltage measurement terminals was then measured by an oscilloscope.From the predetermined current and the voltage change thus measured, theresistivity of these films were then determined. From the resistivityand the thickness of the membrane, conductance was calculated.

[0061] Table 1 shows the conductance of Comparative membranes A, B andC. All the comparative membranes exhibited a conductance of about 0.1S/cm. TABLE 1 Conductance of Comparative membranes A, B and CConductance S/cm Comparative membrane A 0.098 Comparative membrane B0.100 Comparative membrane C 0.103

[0062] Table 2 shows the porosity and conductance of Porous membranes A,B and C.

[0063] In general, when pores are formed in a conductor, the amount ofthe conductor per unit sectional area perpendicular to the direction ofconduction is decreased, lowering the conductance of the conductor. Forexample, a conductor having a porosity of 50% has a density of ½ perunit sectional area and hence an apparent conductance of ½ of that ofthe ion exchange resin free of pores. In other words, an ion exchangeresin having a porosity of 40% exhibits an apparent conductance of 60%of that of an ion exchange resin free of pores and a conductance of 0.06S/cm.

[0064] However, as can be seen in Tables 1 and 2, even an ion exchangeresin having pores formed therein exhibits a conductance equal to orhigher than that of an ion exchange resin having no pores formedtherein. The correction of porosity shows that the conductance of theion exchange resin is enhanced by 50% or more. The details of thismechanism are unknown. It is known that the protonic conductance of anion exchange resin depends on the water content of the ion exchangeresin. The specific phenomenon of conductance of the ion exchange resinhaving pores is presumably attributed to the rise in the surface area ofthe ion exchange resin and the area of the ion exchange resin in contactwith water caused by the porosity of the ion exchange resin.

[0065] As mentioned above, the formation of pores in an ion exchangeresin which is a constituent and covering material of the catalyst layerprovided unexpected results that enhancement is given in not only gaspermeability but also protonic conductance. TABLE 2 Porosity andconductance of Porous membrane A, B and C Porosity Conductance % S/cmPorous membrane A 90 0.070 Porous membrane B 70 0.100 Porous membrane C40 0.112

EXAMPLE 1

[0066] The present invention will be further described in connectionwith FIG. 6 illustrating a production process according to a firstexample of the present invention.

[0067] At the first step, a coating film is formed on a previouslyprepared catalyst layer precursor from an ion exchange resin solution.The formation of the coating film was carried out by coating.

[0068] At the second step, the catalyst layer precursor is soaked in anorganic solvent having a polar group other than alcoholic hydroxyl groupbefore the ion exchange resin solution with which it had beenimpregnated at the first step was dried. During this procedure, thecovering ion exchange resin solution was solidified and rendered porous.

[0069] At the third step, the catalyst layer precursor was allowed todry at room temperature to obtain a gas diffusion electrode provided ina catalyst layer with an ion exchange resin having pores.

[0070] A process for the preparation of the gas diffusion electrodeaccording to the present invention and the electrode assembly comprisingsame will be described in detail.

[0071] At the foregoing first step, as the ion exchange resin solutionthere was used a 9 wt-% Nafion solution obtained by heating andconcentrating “5 wt-% Nafion solution”, which is a trade name of AldrichChemical Inc. of USA, at 60° C.

[0072] The catalyst layer precursor was prepared by applying to a waterrepellent carbon paper a pasty aqueous mixture prepared by adding apolytetrafluoroethylene (PTFE) to a carbon-supported platinum catalysthaving 30 wt-% platinum supported therein in an amount of 30 wt-%, andthen drying the coated material. The gas diffusion electrode thusprepared had a size of 5 cm×5 cm and a platinum content of 0.5 mg/cm².

[0073] The catalyst layer precursor thus prepared was coated with a 9wt-% Nafion solution. The coated amount was about 0.5 mg/cm² ascalculated in terms of dried solid content of Nafion.

[0074] At the second step, as the organic solvent having a polar groupother than alcoholic hydroxyl group there was used n-butyl acetate. Thecatalyst layer precursor thus coated with Nafion solution wasimmediately soaked in n-butyl acetate where it was then allowed tostand.

[0075] At the subsequent third step, the catalyst layer precursor waswithdrawn from n-butyl acetate, and then thoroughly allowed to dry atroom temperature. Thus, a gas diffusion electrode provided in a catalystlayer with an ion exchange resin having pores of the present inventionwas prepared. The gas diffusion electrode thus prepared will behereinafter referred to as “gas diffusion electrode A”. The diagram(electron microphotograph) illustrating the surface conditions of thecatalyst layer in the gas diffusion electrode A is shown in FIG. 7. Theblack portion indicates a carbon-supported platinum catalyst while thewhite portion indicates an ion exchange resin having pores.

[0076] The solid polymer electrolyte membrane was pressed interposedbetween two sheets of the gas diffusion electrodes A at a temperature of130° C. under a pressure of 50 kg/cm² for 2 minutes to obtain anelectrode assembly A. As the solid polymer electrolyte membrane therewas used Nafion 115, which is a trade name of Du Pont Inc. of USA.

[0077] A solid polymer electrolyte fuel cell A (hereinafter simplyreferred to as “cell A”) was prepared from the electrode assembly A.

EXAMPLE 2

[0078] A gas diffusion electrode was prepared from a 13 wt-% Nafionsolution prepared from a commercially available 5 wt-% Nafion solutionand the catalyst layer precursor prepared in Example 1 in the samemanner as in Example 1. The amount of the platinum catalyst and Nafionincorporated in the catalyst layer was the same as in the gas diffusionelectrode A prepared in Example 1. The gas diffusion electrode thusprepared will be hereinafter referred as “gas diffusion electrode B”according to the present invention.

[0079] The gas diffusion electrode B was then bonded to both sides ofNafion 115 membrane, which is a trade name of Du Pont Inc. of USA, as asolid polymer electrolyte membrane in the same manner as in Example 1 toprepare an electrode assembly B. The bonding was carried out by pressingthe combination at a temperature of 130° C. under a pressure of 50kg/cm² for 2 minutes in the same manner as in Example 1.

[0080] A solid polymer electrolyte fuel cell B (hereinafter simplyreferred to as “cell B”) was prepared from the electrode assembly B.

EXAMPLE 3

[0081] A gas diffusion electrode was prepared from a 21 wt-% Nafionsolution prepared from a commercially available 5 wt-% Nafion solutionand the catalyst layer precursor prepared in Example 1 in the samemanner as in Example 1. The amount of the platinum catalyst and Nafionincorporated in the catalyst layer was the same as in the gas diffusionelectrode A prepared in Example 1. The gas diffusion electrode thusprepared will be hereinafter referred as “gas diffusion electrode C”according to the present invention.

[0082] The gas diffusion electrode C was then bonded to both sides ofNafion 115 membrane, which is a trade name of Du Pont Inc. of USA, as asolid polymer electrolyte membrane in the same manner as in Example 1 toprepare an electrode assembly C. The bonding was carried out by pressingthe combination at a temperature of 130° C. under a pressure of 50kg/cm² for 2 minutes in the same manner as in Example 1.

[0083] A solid polymer electrolyte fuel cell C (hereinafter simplyreferred to as “cell C”) was prepared from the electrode assembly C.

[0084] The gas diffusion electrodes A, B and C had differentconcentrations of covering Nafion solution and hence different diametersof pores in the ion exchange resin constituting the catalyst layer. Theresults of observation of surface conditions by an electron microscopeshow that these pore diameters are almost the same as that of pores inthe porous film prepared from Nafion solutions having variousconcentrations shown in Experiment 1. In other words, the ion exchangeresin having pores constituting the catalyst layer in these gasdiffusion electrodes had a pore diameter of from 0.02 to 5.0 μm and aporosity of not less than 40%.

EXAMPLE 4

[0085] A production process according to the fourth example of thepresent invention will be described hereinafter.

[0086] At the first step, an ion exchange resin and a polymer compoundincompatible therewith (hereinafter referred to as “second polymercompound”) were dissolved in an organic solvent capable of dissolvingthe polymer compound therein to prepare a second polymer compoundsolution.

[0087] At the second step, the ion exchange resin solution and thesecond polymer compound solution were mixed with thorough stirring toprepare a mixed dispersion.

[0088] At the third step, the catalyst layer in a gas diffusionelectrode comprising a catalyst layer free of ion exchange resin and agas diffusion layer which had been previously prepared was provided withthe previously mentioned mixed dispersion.

[0089] At the fourth step, the gas diffusion electrode having itscatalyst layer provided with the previously mentioned mixed dispersionwas dried to remove the solvent from the mixed dispersion. Thus, a filmhaving the ion exchange resin present incompatible with the secondpolymer compound or having the second polymer compound dispersed in theion exchange resin was formed.

[0090] At the fifth step, the second polymer compound dispersed in theion exchange resin was eluted with a solvent capable of dissolving onlythe second polymer compound therein to remove the second polymercompound from the ion exchange resin film.

[0091] At the sixth step, the previously mentioned gas diffusionelectrode was dried to form an ion exchange resin film having pores onthe catalyst layer.

[0092] At the foregoing first step, a 0.5 wt-% THF solution of PVC wasprepared from tetrahydrofuran (hereinafter referred to as “THF”) as asolvent and a polyvinyl chloride (hereinafter referred to as “PVC”) as asecond polymer compound.

[0093] At the foregoing second step, as the ion exchange resin solutionthere was used a “5% Nafion solution”, which is a trade name of AldrichChemical Inc. of USA. The 5% Nafion solution and the 0.5 wt-% THFsolution of PVC prepared at the first step were measured out in the sameamount, and then mixed with thorough stirring to prepare an opaque mixeddispersion.

[0094] At the foregoing third step, the foregoing opaque mixeddispersion was applied to the same catalyst layer as prepared inExample 1. The coated amount was about 0.5 mg/cm² as calculated in termsof dried solid content of Nafion.

[0095] At the foregoing fourth step, the gas diffusion electrode havingits catalyst layer provided with the previously mentioned mixeddispersion was kept at a temperature of 60° C. for 12 hours tosufficient dryness.

[0096] At the foregoing fifth step, the foregoing dried gas diffusionelectrode was soaked in THF for 4 hours during which PVC was elutedwhile being permeated by THF.

[0097] At the foregoing sixth step, the foregoing gas diffusionelectrode from which PVC had been eluted was dried. The gas diffusionelectrode thus prepared will be hereinafter referred to as “gasdiffusion electrode D” according to the present invention. The gasdiffusion electrode D was then bonded to a solid polymer electrolytemembrane to obtain an electrode assembly D. As the solid polymerelectrolyte membrane there was used Nafion 115, which is a trade name ofDu Pont Inc. of USA. In some detail, the ion exchange resin membrane waspressed interposed between two sheets of the gas diffusion electrodes Dat a temperature of 130° C. under a pressure of 50 kg/cm² for 2 minutesso that the gas diffusion electrode was bonded to both sides of the ionexchange resin membrane.

[0098] A solid polymer electrolyte fuel cell D (hereinafter simplyreferred to as “cell D”) was prepared from the electrode assembly D.

Comparative Example 1

[0099] A mixture of a carbon-supported platinum catalyst having 30% ofplatinum supported thereon, a polytetrafluoroethylene and a 5% Nafionsolution was prepared. The mixture thus prepared was then applied to acarbon paper rendered water repellent by PTFE to prepare a gas diffusionelectrode. The structure of the gas diffusion electrode was prepared asto have the same composition as in Example 1. The applied amount ofplatinum was 0.5 mg/cm², and the applied amount of Nafion was 0.5mg/cm². The prior art gas diffusion electrode thus prepared will behereinafter referred to as “gas diffusion electrode E”.

[0100] The gas diffusion electrode E was then bonded to a solid polymerelectrolyte membrane to obtain an electrode assembly E. As the solidpolymer electrolyte membrane there was used Nafion 115, which is a tradename of Du Pont Inc. of USA. In some detail, the ion exchange resinmembrane was pressed interposed between two sheets of the gas diffusionelectrodes E at a temperature of 130° C. under a pressure of 50 kg/cm²for 2 minutes so that the gas diffusion electrode was bonded to bothsides of the ion exchange resin membrane.

[0101] A solid polymer electrolyte fuel cell E (hereinafter simplyreferred to as “cell E”) was prepared from the electrode assembly E.

[0102] (Experiment 2)

[0103] Hydrogen gas as fuel gas and oxygen gas as oxidizing gas weresupplied into the cells A, B, C and D according to the present inventionand the comparative cell E at the atmosphere. In this manner, thesecells were measured for current density-cell voltage characteristics.The operating conditions will be given below.

[0104] Operating temperature: 80° C.

[0105] Oxygen humidity temperature: 75° C.

[0106] Hydrogen humidity temperature: 75° C.

[0107] Percent use of oxygen: 50%

[0108] Percent use of hydrogen: 70%

[0109]FIG. 8 illustrates the cell voltage-current density characteristiccurve of the cells A, B, C and D according to the present invention andthe comparative cell E. As can be seen in FIG. 8, the cells A, B, C andD comprising a gas diffusion electrode provided with a catalyst layerhaving pores exhibit a smaller cell voltage drop at a high currentdensity than the comparative cell E provided with the comparative ionexchange resin free of pores and hence excellent polarizationcharacteristics.

[0110] It was also made obvious that by covering a catalyst layer by anion exchange resin having pores, the resulting gas diffusion electrodeand solid polymer electrolyte fuel cell comprising same exhibitremarkable improvement in characteristics over the prior art knownproducts. It was thus confirmed that the provision of the catalyst layerwith the ion exchange resin having pores exerts an effect of enhancingthe power of the solid polymer electrolyte fuel cell.

EXAMPLE 5

[0111] A process for the production of a solid polymer electrolytemembrane provided with pores according to the present invention will bedescribed hereinafter.

[0112] At the first step, the concentration of a solution of an ionexchange resin in a solvent containing an alcohol was adjusted.

[0113] At the second step, the ion exchange resin solution theconcentration of which had been adjusted at the previous step wasapplied to a membrane-forming material having an excellentreleasability.

[0114] At the third step, the membrane-forming material coated with anion exchange resin was soaked in an organic solvent having a polar groupother than alcoholic hydroxyl group so that the ion exchange resin wassolidified and rendered porous.

[0115] At the fourth step, the membrane-forming material was withdrawnfrom the organic solvent used at the previous step, and then dried.

[0116] A process for the preparation of a solid polymer electrolytemembrane having pores according to the present invention and a solidpolymer electrolyte fuel cell comprising same will be described indetail hereinafter.

[0117] At the foregoing first step, a 31 wt-% Nafion solution wasprepared by heating and concentrating a 5 wt-% Nafion solution, which isa trade name of Aldrich Chemical Inc. of USA, as an ion exchange resinsolution at a temperature of 60° C.

[0118] At the foregoing second step, as the membrane-forming materialhaving an excellent releasability there was used a fluorinic polymercompound sheet. This sheet was disposed on a flat surface such as on aglass plate during use. For coating, a doctor blade with its gapadjusted to 0.15 mm was used. In this manner, the 31 wt-% Nafionsolution was applied to the membrane-forming material.

[0119] At the foregoing third step, the membrane-forming material thuscoated with the Nafion solution was immediately soaked in n-butylacetate as an organic solvent having a polar group other than alcoholichydroxyl group where it was then allowed to stand for 20 minutes.

[0120] At the foregoing fourth step, the membrane-forming material waswithdrawn from n-butyl acetate used at the previous step, and thenallowed to dry at room temperature to obtain a solid polymer electrolytemembrane provided with pores. The solid polymer electrolyte membranethus obtained will be hereinafter referred to as “membrane F”.

[0121] The membrane F which is a solid polymer electrolyte membraneprovided with pores had a thickness of 30 μm and a porosity of 10%. As aresult of observation of surface conditions by an electron microscope,it was found that the membrane F had pores having a diameter of from0.02 to 1.0 μm formed therein. The membrane F was measured forconductance in the same manner as in Experiment 1. The results were0.115 S/cm.

[0122] A process for the preparation of a gas diffusion electrode-solidpolymer electrolyte membrane assembly will be described hereinafter. Asa gas diffusion electrode there was used the gas diffusion electrode Bhaving a diameter of 1.5 cm according to the present invention preparedin Example 2. As a solid polymer electrolyte membrane there was usedmembrane F having a diameter of 3.0 cm. In some detail, the membrane Fwas pressed interposed between two sheets of the gas diffusionelectrodes B at a temperature of 130° C. under a pressure of 50 kg/cm²for 2 minutes to prepare a gas diffusion electrode-solid polymerelectrolyte membrane assembly. A solid polymer electrolyte fuel cell F(hereinafter simply referred to as “cell F”) was then prepared from thegas diffusion electrode-solid polymer electrolyte membrane assembly.

Comparative Example 2

[0123] A 31 wt-% Nafion solution was applied to a membrane-formingmaterial by means of a doctor blade with its gap adjusted to 0.18 mm inthe same manner as in Example 5, and then directly allowed to dry toprepare a uniform solid polymer membrane. The solid polymer electrolytemembrane thus obtained will be hereinafter referred to as “membrane G”.

[0124] The membrane G, which is a solid polymer electrolyte membraneprovided with pores, had a thickness of 30 μm. The membrane G wasmeasured for conductance in the same manner as in Experiment 1. Theresults were 0.101 S/cm.

[0125] As a gas diffusion electrode there was used the prior art knowngas diffusion electrode E having a diameter of 1.5 cm prepared inComparative Example 1. As a solid polymer electrolyte membrane there wasused a membrane G having a diameter of 3.0 cm. In some detail, themembrane G was pressed interposed between two sheets of the gasdiffusion electrodes E at a temperature of 130° C. under a pressure of50 kg/cm² in the same manner as in Example 5 for 2 minutes. Thus, a gasdiffusion electrode-solid polymer electrolyte membrane assembly wasprepared. A comparative solid polymer electrolyte fuel cell G(hereinafter simply referred to as “cell G”) was then prepared from thegas diffusion electrode-solid polymer electrolyte membrane assembly.

[0126] (Experiment 3)

[0127] The cells F and G were measured for cell voltage-current densitycharacteristics in the same manner as in Experiment 2. As a result, thetwo cells showed almost the same characteristics. Further, the two cellswere subjected to life test at a temperature of 60° C. As a result, thecell F according to the present invention exhibited better results. Thisis presumably because the solid polymer electrolyte membrane providedwith an ion exchange resin having pores has a good water retention.

[0128] This result shows that the use of a solid polymer electrolytemembrane having pores makes it possible to form a solid polymerelectrolyte fuel cell having a prolonged life. Further, the fact thatthe cell F can exhibit the same characteristics as the cell G despiteits great thickness of ion exchange resin membrane also shows that theformation of pores in a solid polymer electrolyte membrane enhances theconductance of the solid polymer electrolyte fuel cell, making itpossible to provide the resulting solid polymer electrolyte fuel cellwith an enhanced output.

[0129] Industrial Applicability

[0130] As mentioned above, the gas diffusion electrode according to thepresent invention has pores in the catalyst layer, i.e., ion exchangeresin by which the catalytic substance is covered, making it possible toprevent the carbon-supported platinum catalyst from being excessivelycovered. Further, since the pores are in the form of three-dimensionalnetwork structure, the resulting catalyst layer has a high permeabilityto oxygen or hydrogen, making it possible to facilitate the supply of areaction gas to the catalytic substance. Moreover, since the ionexchange resin has a continuous three-dimensional network structure, asufficient path of migration of proton can be formed. Further, theincrease in the area of contact with water accompanying the increase inthe surface area of the ion exchange resin by the porosity of the ionexchange resin makes it possible to enhance the conductance of the ionexchange resin.

[0131] Further, the production process according to the presentinvention allows the maintenance of electrical contact between thecatalyst particles, making it possible to form a sufficient path ofmigration of electron. Moreover, the solid polymer electrolyte membraneaccording to the present invention exhibits a lowered resistivity and animproved water retention.

[0132] Accordingly, the resulting catalyst layer can be provided with athree-phase boundary also there inside while maintaining sufficientelectron conductivity, capability of supplying reaction gas and protonconductance, making it possible to provide a solid polymer electrolytemembrane-gas diffusion electrode assembly having excellent polarizationcharacteristics. In addition, a solid polymer electrolyte fuel cellhaving a high power density can be provided.

1. A gas diffusion electrode for solid polymer electrolyte fuel cellcomprising a gas diffusion layer and a catalyst layer, characterized inthat said catalyst layer is provided with a catalytic substance and anion exchange resin having pores.
 2. The gas diffusion electrode forsolid polymer electrolyte fuel cell according to claim 1, wherein saidion exchange resin has pores having a diameter of from 0.05 to 5.0 μmand porosity of not less than 40%.
 3. The gas diffusion electrode forsolid polymer electrolyte fuel cell according to claim 1 or 2, whereinsaid ion exchange resin is a perfluorosulfonic acid resin and saidcatalytic substance is a particulate noble metal or carbon having aparticulate noble metal supported thereon.
 4. A process for theproduction of a gas diffusion electrode for solid polymer electrolytefuel cell according to any one of claims 1 to 3, which comprises formingan ion exchange resin coating film on a catalyst layer precursorcomprising at least a catalytic substance from a solution obtained bydissolving an ion exchange resin in a solvent contained in an alcohol,and then soaking the ion exchange resin coating film in an organicsolvent having a polar group other than alcoholic hydroxyl group so thatthe ion exchange resin is solidified and rendered porous.
 5. A solidpolymer electrolyte membrane-gas diffusion electrode assembly comprisinga gas diffusion electrode according to any one of claims 1 to 3 providedon at least one side of a solid polymer electrolyte membrane.
 6. A solidpolymer electrolyte fuel cell comprising a solid polymer electrolytemembrane-gas diffusion electrode assembly according to claim
 5. 7. Asolid polymer electrolyte membrane comprising an ion exchange resin as aconstituent and having pores.
 8. The solid polymer electrolyte membraneaccording to claim 7, having a pore diameter of from 0.02 to 1.0 μm anda porosity of not less than 10%.
 9. The solid polymer electrolytemembrane according to claim 7 or 8, wherein said ion exchange resin is aperfluorosulfonic acid resin.
 10. A process for the production of asolid polymer electrolyte membrane according to any one of claims 7 to9, which comprises soaking a solution comprising an ion exchange resindissolved in a solvent containing an alcohol in an organic solventhaving a polar group other than alcoholic hydroxyl group so that saidion exchange resin is solidified and rendered porous to form an ionexchange resin membrane having pores.
 11. A solid polymer electrolytefuel cell comprising a solid polymer electrolyte membrane according toany one of claims 7 to 9.